The hexatic phase is an intermediate stage in the melting process of a 2D crystal due to topological defects. Recently, this exotic phase was experimentally identified in the vortex lattice of 2D weakly disordered superconducting MoGe by scanning tunneling microscopic measurements. Here, we study this vortex state by the Nernst effect, which is an effective and sensitive tool to detect vortex motion, especially in the superconducting fluctuation regime. We find a surprising Nernst sign reversal at the melting transition of the hexatic phase. We propose that they are a consequence of vortex dislocations in the hexatic state which diffuse preferably from the cold to hot.
We describe the near-field scanning microwave microscopy of microwave devices on a length scale much smaller than the wavelength used for imaging. Our microscope can be operated in two possible configurations, allowing a quantitative study of either material properties or local electric fields.
We have used 2 ns pulses of current to read out the states of a Nb/AlOx/Nb dc SQUID phase qubit at 25 mK. Plotting the number of escape events that occur versus the size of the current pulse reveals a series of steps corresponding to the occupancy of different energy levels. After calibrating the measurement pulses, we fit these steps to determine the population in each level. Rabi oscillations were viewed using this technique. The single-shot measurement fidelity was theoretically analysed and the optimal single pulse in our device was determined.
We describe a technique for initializing the flux state of an inductively isolated Josephson junction, fulfilling an essential requirement for using the device as a qubit. By oscillating the applied magnetic flux with a specified amplitude and offset, we can select any of the allowed long-lived metastable flux states. We applied this technique to $\mathrm{Nb}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Nb}$ and $\mathrm{Al}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Al}$ devices with from 10 to over 100 distinct flux states at temperatures as low as $25\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. In a ten-state system with an initial probability $p=0.13$ to be in the desired flux state, we achieved $p=0.999\phantom{\rule{0.2em}{0ex}}96$ after 50 oscillations at $22.6\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$. The technique is generally applicable to other systems with multiple metastable wells (including rf SQUIDs), requires no additional readout or bias lines, involves minimal energy dissipation, and appears to scale favorably with the number of qubits.
Ir-based materials have drawn much attention due to the observation of insulating phase believed to be driven by spin-orbit coupling, while Ir $5d$ states are expected to be weakly correlated due to their large orbital extensions. ${\mathrm{IrO}}_{2}$, a simple binary material, shows a metallic ground state which seems to deviate from the behavior of most other Ir-based materials and varied predictions in this material class. We studied the electronic structure of ${\mathrm{IrO}}_{2}$ at different temperatures, employing high-resolution photoemission spectroscopy with photon energies spanning from ultraviolet to hard $x$-ray range. Experimental spectra exhibit a signature of enhancement of Ir-O covalency in the bulk compared to the surface electronic structure. The branching ratio of the spin-orbit split Ir core-level peaks is found to be larger than its atomic values and it enhances further in the bulk electronic structure. Such deviation from the atomic description of the core-level spectroscopy manifests the enhancement of the orbital moment due to the uncompensated electric field around Ir sites. The valence-band spectra could be captured well within the density functional theory. The photon energy dependence of the features in the valence-band spectra and their comparison with the calculated results show dominant Ir $5d$ character of the features near the Fermi level; O $2p$ peaks appear at higher binding energies. Interestingly, the O $2p$ contributions of the feature at the Fermi level are significant, and it enhances at low temperatures. This reveals an orbital selective enhancement of the covalency with cooling, which is an evidence against the purely spin-orbit coupling based scenario proposed for these systems.
Over the years welding has developed and occupied an important position and has become the most suitable method for joining materials. Hand in hand with the developments in the field of materials, welding has also developed to accommodate new consumables.
We describe the use of a cryogenic near-field scanning microwave microscope to image microwave electric fields from superconducting and normal-metal microstrip resonators. The microscope employs an open-ended coaxial probe and operates from 77 to 300 K in the 0.01-20 GHz frequency range with a spatial resolution of about 200 mm. We describe the operation of the system and present microwave images of Cu and Tl2Ba2CaCu2O8 microstrip resonators, showing standing wave patterns at the fundamental and second harmonic frequencies.
Ir-based materials have drawn much attention due to the observation of insulating phase believed to be driven by spin-orbit coupling while Ir 5$d$ states are expected to be weakly correlated due to their large orbital extensions. IrO$_2$, a simple binary material, shows metallic ground state which seems to deviate from the behavior of most other Ir-based materials and varied predictions in these material class. We studied the electronic structure of IrO$_2$ at different temperatures employing high resolution photoemission spectroscopy with photon energies spanning from ultraviolet to hard $x$-ray range. Experimental spectra exhibit a signature of enhancement of Ir-O covalency in the bulk compared to the surface electronic structure. The branching ratio of the spin-orbit split Ir core level peaks is found to be larger than its atomic values and it enhances further in the bulk electronic structure. Such deviation from the atomic description of the core level spectroscopy manifests the enhancement of the orbital moment due to the solid state effects. The valence band spectra could be captured well within the density functional theory. The photon energy dependence of the features in the valence band spectra and their comparison with the calculated results show dominant Ir 5$d$ character of the features near the Fermi level; O 2$p$ peaks appear at higher binding energies. Interestingly, the O 2$p$ contributions of the feature at the Fermi level is significant and it enhances at low temperatures. This reveals an orbital selective enhancement of the covalency with cooling which is an evidence against purely spin-orbit coupling based scenario proposed for these systems.
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